Network News

LONDON, July 21 -- Four small explosions shut down the city's public transit system at lunchtime Thursday and sent panicked passengers fleeing for safety in an echo of the deadly suicide strikes of two weeks ago. Police reported one injury.

Just as on July 7, men carrying bombs concealed in backpacks hit three subway trains and a double-decker bus in quick succession. Initial investigation suggested that explosives in the bags only partially detonated, officials said, preventing carnage on a scale of the previous attacks, which killed at least 56 people, including the attackers.

Witnesses described sharp bangs that created acrid clouds of thick white smoke. Attackers were seen fleeing on foot. In at least one station, passengers chased after a man they believed to be the culprit, but he got away.

Investigators said they were pursuing several unidentified suspects, but gave no details. Two men were arrested, but were quickly released, officials said. Authorities acknowledged that, as with the previous bombings, they had had no intelligence warning of the attacks, though the city is on a high state of alert.

Backpacks left behind by the men, numerous witness accounts and footage from dozens of security cameras gave authorities a huge trove of evidence to sift through. But police said they did not know whether the attackers were linked to the four young British Muslims suspected of carrying out the July 7 bombings or were amateur copycats who failed to detonate lethal amounts of explosives.

"Clearly the intention must have been to kill," Ian Blair, commissioner of the Metropolitan Police, told reporters.

"There is a resonance here, isn't there?" said Blair, referring to the July 7 attacks. "Whether or not this is directly connected in the sense of being carried out by the same group of people, however loosely knit -- that is going to take just a little bit longer" to determine.

Officials refused to describe the explosives used, although they said they were investigating whether the materials were similar to those of the July 7 attacks.

One British source said Londoners had been lucky because some of the explosives apparently failed to detonate. Had they all gone off, the source said, they could have caused casualties and damage similar to those in the original blasts.

The attacks badly rattled a city where people had been starting to regain confidence after the past bombings. Transit systems were shut down in response for much of the day, forcing thousands of workers to make their way home on foot.

The first detonation occurred just before 12:25 p.m. on a subway car on the Hammersmith and City Line that was passing through an aboveground station at Shepherd's Bush in west London. Passengers told reporters that a muscular-looking black man ran from the platform, lowered himself down a wall and bolted through backyards.

Five minutes later, a second detonation occurred on the Northern Line at the Oval station, south of the Thames River. Witnesses described it as a sharp popping sound like champagne corks and said it came from a backpack that an Asian man who looked about 19 had dumped inside a train.